1. Field of the Invention
The present invention relates to a method of controlling the steering of rear wheels of a motor vehicle having steerable front and rear wheels.
2. Discussion of Relevant Art
There is known a motor vehicle having a frontwheel steering device for steering the front wheels according to the angle through which a steering wheel is turned, a rear-wheel steering device for steering the rear wheels with an actuator, and a control device responsive to the speed of travel of the motor vehicle and the steering angle of the steering wheel for controlling the operation of the actuator to steer the rear wheels in a direction which is selectively the same as or opposite to the direction in which the front wheels are turned. Such a motor vehicle is disclosed in Japanese Laid-Open patent Publication Nos. 59-70261, 59-77968, 59-77970, 59-77971, 59-81257, and 59-81258, for example.
The motor vehicle with steerable front and rear wheels is advantageous for the following reasons. Initially, when a front-wheel-steered motor vehicle with rear wheels that are not steerable makes an S-shaped turn at a relatively high speed, the extent to which the motor vehicle has to swing is large since the center of gravity G of the motor vehicle is laterally shifted at a large angle .theta., as shown in FIG. 1(a). Quite differently, when a motor vehicle with steerable front and rear wheels goes along the same S-shaped turn at a relatively high speed, the front and rear wheels are steered in the same direction as shown in FIG. 1(b) to substantially eliminate the angle at which the center of gravity G is laterally shifted, thereby minimizing the swinging movement of the motor vehicle.
In the motor vehicle with the steerable front and rear wheels, therefore, the swinging movement of the center of gravity G about a vertical axis can be held to a minimum when the motor vehicle makes a turn at a relatively high speed. As a result, the motor vehicle has improved dynamic effects, i.e., the response delay in the motion of the motor vehicle is reduced, and the tires remain stuck to the road surface stably over a greater speed range on curving roads for giving the motor vehicle higher limitations in performance and maneuverability.
Furthermore, when travelling along a curve at a low speed, the front-wheel-steered motor vehicle has a large difference between the radii of turns followed by the front and rear inner wheels, as shown in FIG. 2(a), and hence the radius of turns that the motor vehicle can make cannot be reduced below a certain limitation. With the motor vehicle having the steerable front and rear wheels, however, the front and rear wheels are steered in opposite directions as shown in FIG. 2(b), with the result that static effects can be produced, i.e., the radius of turns followed by the motor vehicle is smaller, and the difference l between the radii of turns of the front and rear inner wheels is substantially eliminated.
The principle of controlling the steering of the rear wheels of the motor vehicle with the steerable front and rear wheels is therefore that the lateral shift of the center of gravity G at the time the motor vehicle is turned is reduced to zero.
Conventional methods of controlling the steering of the rear wheels include a method in which the rear wheels are steered through an angle which is predetermined according to the speed of travel of the motor vehicle or the steering angle of the steering wheel, and a method in which an actual lateral shift of the center of gravity is directly measured and the rear wheels are steered on the basis of the measured lateral shift of the center of gravity.
The latter method is capable of measuring the lateral shift of the center of gravity, but is not practically feasible because its reliability is low and the system for carrying out the method is large in size and expensive.